lang: funcs: core: Add value functions

This adds a new series of "get*" functions which can read values from
the associated "value" resources. The key name of the function must
match the name value of the resource for things to work.

Type unification isn't yet perfect in these scenarios, so you should use
casually and with caution.

lang/funcs/core/core.go

@@ -36,6 +36,7 @@ import (
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/strings"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/sys"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/test"
+	_ "github.com/purpleidea/mgmt/lang/funcs/core/value"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/world"
 )